Geology studies the composition and structure, both internal and superficial, of the Earth. In doing so, it investigates the processes by which it has evolved over geological time. Layer after stratum the geologists walk... and from time to time something does not fit, a discordance, as if the story of the future of the Earth suddenly skipped several chapters.
Among the discordances that geologists have seen there is one especially famous. They call it the Great Discord. This is an exceptional example in which relatively young sedimentary rock strata are superimposed on much older sedimentary or crystalline strata (almost 2 billion years old). This is what John Wesley Powell found in the Grand Canyon of the United States in 1869.
Imagine a book with a central section ripped out. The Great Unconformity is that gap in Earth's history."
The geological time interval that elapsed between those strata is 200 million years, wide enough to have allowed the oldest rocks to rise to form mountains, which were subsequently eroded. Science has been trying for years to explain the Great Unconformity, the absence of those 200 million years, and now Australian researchers claim to have found a plausible theory.
Why is the 200 million year record missing?
Since that distant 1869, various hypotheses have been proposed to explain the time jump in the strata of the Grand Canyon. There is a general consensus that a combination of events could have led to a phenomenon of such magnitude.
It is believed that during one or more of the periods of glacial climate in Earth's history, the planet's surface was almost completely frozen, with average temperatures of -50 °C. It is the paleoclimatic hypothesis of generalized glaciation or superglaciation, also known as "Snowball Earth." So in the Neoproterozoic the planet sailed through space like a great white ball of ice.
According to some geologists, the areas that suffered that glaciation approximately coincide with those where the Great Unconformity is located today. As glaciers move, they drag and erode the sediments that cover the underlying rock. This would explain how a large portion of rock was removed from large areas during the same period.
Other hypotheses speak of the formation of rifts (constructive boundaries between tectonic plates in the axis of the oceanic ridges), the activity of mantle plumes (columns of material from the mantle that are supposed to exist under the Earth's crust) or the drop in sea level. But none of those explanations have been proven.
Processes that occur deep in the Earth
Now, Santosh Dhungana and Nicolas Flament, geologists at the University of Wollongong, in Australia, propose that the disappearance of this enormous surface of strata in the Grand Canyon of the Colorado was caused by deep movements in the Earth's mantle. The Great Unconformity would be a superficial, first-order manifestation of processes that occur in the deep interior of the Earth. The article explaining it has just been published in the journal Nature.
"Sometimes what's missing can tell us more about Earth's history than what we can see."
"Imagine a book with a large central section torn out. The Great Unconformity is that gap in Earth's history: the narrative jumps from the first chapters directly to a much later part of the story," explains Dhungana. "Our research suggests that these pages were not lost by chance," he says.
The study proposes that continental-scale uplift and subsidence were the factors driving the formation and preservation of the Great Unconformity. Simulations of mantle movements used by Dhungana and Flament indicate that parts of the planet were uplifted, exposing themselves to erosion that, after the descent of the continents, eliminated them from the record. That would explain the lack in the record of the terrestrial strata.
"Sometimes what's missing can tell us more about Earth's history than what we can see. A single, long period of uplift, followed by a long period of subsidence, would have been the ideal recipe for removing the rock and preserving the eroded surface," says Flament.
Specifically, the Australian scientists' models point to prolonged continental uplift during the Neoproterozoic, followed by long-term subsidence during the Phanerozoic, in the Laurentia, Baltica and northern China cratons. This scenario of vertical movements described above is ideal for the formation and preservation of the Great Unconformity.
